One of the main categories of engine-powered watercraft comprises watercraft with a planing or semiplaning keel; such keels enable the hull to plane when determined hydrodynamic support conditions are attained, i.e. when the weight of the hull is supported mainly by the dynamic action between the water and the keel plane.
The watercraft thus rises on the water and slides thereon instead of simply floating thereon. These planing conditions occur only at high speed, and it is therefore extremely important to as quickly as possible pass through the transient conditions between the moment in which the hull simply floats, not having as yet attained the speed required for planing, and the moment in which it planes.
It is apparent that the duration of this transient period depends on the ability to fully use the engine power. As these watercraft use a propeller propulsion system, the problem of the duration of the transient period corresponds to solving the problem of optimising the propeller operation.
Increasing the propeller speed is known to create certain problems, deriving from the fact that to develop maximum thrust the water must slide over the propeller blades with as little turbulence as possible, so that a vacuum is created on its front face. If the propeller is accelerated such that this vacuum is less than that exerted overall by the water, the flow alters and gives rise to the phenomenon of cavitation, which results in a rapid reduction in the torque absorbed by the propeller and an even greater reduction in the developed thrust.
The propeller thrust is greater if the volume and speed of the mass of water which traverse the propeller disc per unit of time are higher and its acceleration is lower; therefore many types of propeller and many types of propeller-engine coupling have been studied to optimise the propulsion efficiency of a watercraft as a function of its keel shape and speed characteristics.
In the case of planing or semiplaning keels, propellers of high speed type are frequently used, to ensure that the high speed required for planing is obtained.
Before the watercraft has reached the speed required for planing, such propellers are not operating in optimum conditions because they are rotating at low r.p.m. and cannot be raised rapidly to the required r.p.m.. Current marine propulsion units do not in fact have change-speed gears so that the propeller drive torque can only adapt to the resistant torque if the drive torque is always greater than the resistant torque. This means that the engine r.p.m. must be gradually increased. Only when the planing speed for the watercraft is reached will the engine operate under optimum maximum drive torque conditions.
The methods used up to the present time to solve this problem are not however totally satisfactory and new solutions are continuously sought by experts in the art, specially for high-speed or competition crafts for which the time required for attaining planing speed is very important.